Turbo Multiuser Detection Research Articles

Bit-Interleaved Coded SCMA With Iterative Multiuser Detection: Multidimensional Constellations Design

This paper investigates the constellation/codebook design of a promising uplink multiple access technique, sparse code multiple access (SCMA), proposed for the fifth generation mobile networks. The application of bit-interleaved coded modulation with iterative multiuser detection is considered for uplink SCMA over fading channels. Extrinsic information transfer chart is used to aid the analysis and the design of multidimensional constellations, and the design criteria for multidimensional constellations and labelings optimization are thus established. Furthermore, a new and simple approach of multi-stage optimization for the multidimensional constellation design is proposed for SCMA, to improve the bit-error rate performance and alleviate the complexity of turbo multiuser detection. Numerical and simulation results are also provided to demonstrate the performance and verify the efficiency of the proposed scheme, compared with the state of the art.

Performance enhancement of MC-CDMA system through novel sensitive bit algorithm aided turbo multi user detection.

Multi carrier code division multiple access (MC-CDMA) system is a promising multi carrier modulation (MCM) technique for high data rate wireless communication over frequency selective fading channels. MC-CDMA system is a combination of code division multiple access (CDMA) and orthogonal frequency division multiplexing (OFDM). The OFDM parts reduce multipath fading and inter symbol interference (ISI) and the CDMA part increases spectrum utilization. Advantages of this technique are its robustness in case of multipath propagation and improve security with the minimize ISI. Nevertheless, due to the loss of orthogonality at the receiver in a mobile environment, the multiple access interference (MAI) appears. The MAI is one of the factors that degrade the bit error rate (BER) performance of MC-CDMA system. The multiuser detection (MUD) and turbo coding are the two dominant techniques for enhancing the performance of the MC-CDMA systems in terms of BER as a solution of overcome to MAI effects. In this paper a low complexity iterative soft sensitive bits algorithm (SBA) aided logarithmic-Maximum a-Posteriori algorithm (Log MAP) based turbo MUD is proposed. Simulation results show that the proposed method provides better BER performance with low complexity decoding, by mitigating the detrimental effects of MAI.

Performance Analysis of the Downlink CP-WCDMA System with Turbo Code in Frequency Domain

Modern wireless communications require an efficient spectrum usage and high channel capacity and high throughput. Turbo code, linear equalizers, multi-user detection and wideband code-division multiple access (WCDMA) are possible solutions to achieve spectral efficiency, high channel capacity, eliminate MAI, eliminate ISI and robustness against frequency selective fading. In this paper, we combine all these techniques and investigate BER performance. We propose a low complexity receiver structure for Single-input Single-output downlink cyclic prefix CP-WCDMA systems. It employs frequency domain interference cancellation schemes to mitigate the interference caused by the multipath fading channel. Also, the proposed scheme is developed for the downlink Turbo code CP-WCDMA system to maximize the throughput of the proposed system.

Evolutionary-Algorithm-Assisted Joint Channel Estimation and Turbo Multiuser Detection/Decoding for OFDM/SDMA

The development of evolutionary algorithms (EAs), such as genetic algorithms (GAs), repeated weighted boosting search (RWBS), particle swarm optimization (PSO), and differential evolution algorithms (DEAs), have stimulated wide interests in the communication research community. However, the quantitative performance-versus-complexity comparison of GA, RWBS, PSO, and DEA techniques applied to the joint channel estimation (CE) and turbo multiuser detection (MUD)/decoding in the context of orthogonal frequency-division multiplexing/space-division multiple-access systems is a challenging problem, which has to consider both the CE problem formulated over a continuous search space and the MUD optimization problem defined over a discrete search space. We investigate the capability of the GA, RWBS, PSO, and DEA to achieve optimal solutions at an affordable complexity in this challenging application. Our study demonstrates that the EA-assisted joint CE and turbo MUD/decoder is capable of approaching both the Cramer-Rao lower bound of the optimal CE and the bit error ratio (BER) performance of the idealized optimal maximum-likelihood (ML) turbo MUD/decoder associated with perfect channel state information, respectively, despite imposing only a fraction of the idealized turbo ML-MUD/decoder's complexity.

Turbo Multi-User Detection for OFDM/SDMA Systems Relying on Differential Evolution Aided Iterative Channel Estimation

A differential evolution (DE) algorithm aided iterative channel estimation and turbo multi-user detection (MUD) scheme is proposed for multi-user multi-input multiple-output aided orthogonal frequency-division multiplexing / space-division multiple-access (OFDM/SDMA) systems. The proposed scheme iteratively exchanges the estimated channel information and the detected data between the channel estimator and MUD employing a turbo technique, which gradually improves the accuracy of the channel estimation and the MUD, especially for the first iteration. Quadrature amplitude modulation (QAM) is employed in most wireless standards by virtue of providing a high throughput. However, the optimal maximum likelihood (ML)-MUD becomes extremely complex for employment in QAM-aided multi-user systems. Hence, two different DE aided MUD schemes, the DE aided minimum symbol error rate (MSER)-MUD as well as the discrete DE aided ML-MUD, were developed, and their achievable performance versus complexity was characterized. The simulation results demonstrate that the proposed DE aided channel estimator is capable of approaching the Cramer-Rao lower bound with just two or three iterations. The ultimate bit error rate lower-bound of the single-user additive white Gaussian noise scenario has been approached in the range of E <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">b</sub> / N <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">0</sub> ≥ 10 dB and E <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">b</sub> / N <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">0</sub> ≥ 6 dB for the DE aided MSER-MUD and the discrete DE aided ML-MUD, respectively.

A novel bandwidth efficient SOC-based turbo coding scheme mid reduced complexity MUD for SA-based MC-CDMA systems

Multiuser-detection (MUD), turbo coding and smart-antennas (SA) are powerful techniques for enhancing the performance and capacity of MC-CDMA systems. Among the MUD algorithms, the maximum-likelihood (ML) method has the best performance but its complexity increases exponentially with the number of users and constellation size. In this paper, we first propose a novel bandwidth-efficient-channel-coding-scheme (BECCS) for a super-orthogonal-code (SOC)-based serially concatenated turbo code (SCSOC) so that by using it, the coded system without extra bandwidth significantly improves the performance of an uncoded system over a fading channel. Second, in order to reduce the complexity of the ML-based turbo MUD technique, an ML algorithm based on the sensitive-bits-algorithm (SBA) and a less-complex-norm-approximation (LCNA) based Euclidean distance is proposed for a SCSOC-based BECCS assisted coded MC-CDMA system accompanied by SA techniques at the receiver. Our analytical and simulation results show that from a performance perspective, at BER=10?2, the proposed SCSOC-based BECCS assisted MC-CDMA system performs 4 dB better than SOC-based coded systems. The latter system has 5 dB gain in comparison with an uncoded one, all in the same bandwidth and over fading channels.

A Variational Inference Framework for Soft-In Soft-Out Detection in Multiple-Access Channels

We propose a unified framework for deriving and studying soft-in soft-out (SISO) detection in multiple-access channels using the concept of variational inference. The proposed framework may be used in multiple-access interference (MAI), intersymbol interference (ISI), and multiple-input multiple-output (MIMO) channels. Without loss of generality, we will focus our attention on turbo multiuser detection, to facilitate a more concrete discussion. It is shown that, with some loss of optimality, variational inference avoids the exponential complexity of a posteriori probability (APP) detection by optimizing a closely related, but much more manageable, objective function called variational free energy. In addition to its systematic appeal, there are several other advantages to this viewpoint. First of all, it provides unified and rigorous justifications for numerous detectors that were proposed on radically different grounds, and facilitates convenient joint detection and decoding (utilizing the turbo principle) when error-control codes are incorporated. Second, efficient joint parameter estimation and data detection is possible via the variational expectation maximization (EM) algorithm, such that the detrimental effect of inaccurate channel knowledge at the receiver may be dealt with systematically. We are also able to extend BPSK-based SISO detection schemes to arbitrary square QAM constellations in a rigorous manner using a variational argument.

Joint Turbo Equalization and Multiuser Detection of MC-CDMA Signals With Low Envelope Fluctuations

In this paper, we consider the uplink transmission in multicarrier code-division multiple-access (MC-CDMA) systems. As other multicarrier signals, MC-CDMA signals have high envelope fluctuations and a high peak-to-mean envelope power ratio (PMEPR), which leads to amplification difficulties. This is particularly important for the uplink transmission, since an efficient low-cost power amplification is desirable at the mobile terminals (MTs). Moreover, the transmission over time-dispersive channels destroys the orthogonality between spreading codes, which might lead to significant multiple-access interference levels. To reduce the envelope fluctuations of the transmitted signals, while maintaining the spectral efficiency, the MC-CDMA signal associated to each MT is submitted to a clipping device, followed by a frequency-domain filtering operation. However, the nonlinear distortion effects can be high when an MC-CDMA transmitter with reduced envelope fluctuations is intended (e.g., a small clipping level and/or when successive clipping and filtering operations are employed). In this paper, we define an iterative receiver that jointly performs a turbo multiuser detection and the estimation and cancellation of the nonlinear distortion effects. Our performance results show that the proposed receiver structure allows good performances, very close to the linear receiver ones, even for high system load and/or when a PMEPR as low as 1.7 dB is intended for each MT.

Bit-Level Equalization and Soft Detection for Gray-Coded Multilevel Modulation

This correspondence investigates iterative soft-in-soft-out (SISO) detection in coded multiple access channels, with Gray-coded <i xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">M</i> -ary quadrature amplitude modulation (QAM) for the channel symbols. The proposed solution may be summarized as a generic iterative detection scheme called bit-level equalization and soft detection (BLESD), which is an extension of a unified variational inference framework for binary SISO detection proposed in our prior work. This new strategy fundamentally differs from the conventional symbol detector, in that data symbols are transparent to the new detector. Rather, soft estimates of the bits that make up the symbols are directly and naturally obtained by the detector in terms of posterior probabilities given the channel observation, facilitating efficient message-passing in joint detection and decoding. Case studies that illustrate the applications of the proposed scheme are presented for turbo multiuser detection (MUD) for multiple-access interference (MAI) channels and turbo equalization for inter-symbol interference (ISI) channels.

Frequency-Domain Turbo Equalization and Multiuser Detection for DS-UWB Systems

One of the major challenges in direct sequence-ultra wideband (DS-UWB) receiver design is intersymbol interference (ISI). Several equalization schemes to eliminate ISI in DS-UWB systems have been proposed in the literature. It was shown that frequency-domain (FD) equalization techniques can offer better trade off between performance and complexity compared to time- domain equalization schemes for DS-UWB systems on highly dispersive channels. In this paper, we derive low-complexity FD minimum mean square error turbo equalization schemes for single-user binary phase shift keying (BPSK) and quaternary bi-orthogonal keying (4BOK) DS-UWB systems. For multiuser DS-UWB systems, we combine FD turbo equalization schemes with soft interference cancelation to obtain multiuser FD turbo detectors. The bit error rate performance gain due to turbo detection is shown to be significant, particularly for multiuser DS-UWB systems.

Novel decoding of square QAM modulated mimo systems based on turbo multiuser detection

By introducing the bit-level multi-stream coded Layered Space-Time (LST) transmitter along with a novel iterative MultiStage Decoding (MSD) at the receiver, the paper shows how to achieve the near-capacity performance of the Multiple-Input Multiple-Output (MIMO) systems with square Quadrature Amplitude Modulation (QAM). In the proposed iterative MSD scheme, the detection at each stage is equivalent to multiuser detection of synchronous Code Division Multiple Access (CDMA) multiuser systems with the aid of the binary representation of the transmitted symbols. Therefore, the optimal Soft-Input Soft-Output (SISO) multiuser detection and low-complexity SISO multiuser detection can be utilized herein. And the proposed scheme with low-complexity SISO multiuser detection has polynomial complexity in the number of transmit antennas M, the number of receive antennas N, and the number of bits per constellation point M c . Simulation results demonstrate that the proposed scheme has similar Bit Error Rate (BER) performance to that of the known Iterative Tree Search (ITS) detection.

A turbo SDMA receiver for strongly nonlinearly distorted MC-CDMA signals

This paper considers the use of space division multiple access (SDMA) techniques for the uplink of multi-carrier code division multiple access (MCCDMA) systems, where the transmitted signals face strong nonlinear distortion effects. The signal transmitted by each mobile terminal (MT) is submitted to a nonlinear operation consisting of a clipping device followed by a frequency-domain filtering operation, so as to reduce the envelope fluctuations and peak-to-mean envelope power ratio (PMEPR) while maintaining the spectral occupation of conventional MC-CDMA signals. At the base station, an iterative receiver employing multiple antennas jointly performs turbo multi-user detection and the estimation and cancellation of nonlinear distortion effects. Performance results show that the proposed receiver structures can achieve good performances that are very close to those obtained with linear transmitters, even for high system load and/or when a very low-PMEPR MC-CDMA transmission is intended for each MT.

Hybrid Turbo Multiuser Detection for OFDM Transmission with Spatially-Correlated Channels

This letter proposes a novel hybrid turbo multi-user (MU) detection technique based on group-wise soft interference canceling minimum mean-square error filtering (SC-MMSE) combined with maximum a posteriori (MAP) signal detection for OFDM MU systems. A new user grouping algorithm is developed that exploits the knowledge of the pairwise spatial correlation among the channels of each group, of which aim is to reduce noise enhancement due to the MMSE interference suppression of highly correlated user signals. It is shown that the proposed detector can achieve significant performance improvements over SC-MMSE in the presence of high spatial channel correlation

Impact of Channel Estimation Errors on Multiuser Detection via the Replica Method

For practical wireless DS-CDMA systems, channel estimation is imperfect due to noise and interference. In this paper, the impact of channel estimation errors on multiuser detection (MUD) is analyzed under the framework of the replica method. System performance is obtained in the large system limit for optimal MUD, linear MUD, and turbo MUD, and is validated by numerical results for finite systems.

Optimization of LDPC-coded turbo CDMA systems

We consider the analysis and design of low-density parity-check (LDPC) codes for turbo multiuser detection in multipath code division multiple access (CDMA) channels. We develop techniques for computing the probability density function (pdf) of the extrinsic messages at the output of the soft-input soft-output (SISO) multiuser detectors as a function of the pdf of input extrinsic messages, user spreading codes, channel impulse responses, and signal-to-noise ratios. Of particular interest is the soft interference cancellation plus minimum mean square error (SIC-MMSE) multiuser detector, for which the pdf of the extrinsic messages can be characterized analytically. For the case of additive white Gaussian noise (AWGN) channels, the extrinsic messages can be well approximated as symmetric Gaussian distributed. For the case of asynchronous multipath fading channels, the extrinsic messages can be approximated by a mixture of symmetric Gaussian distributions. We show that the expectation-maximization (EM) algorithm can be used to compute the parameters of this mixture. Using these techniques, we are able to accurately compute the thresholds for LDPC codes and design good irregular LDPC codes. Simulation results are in good agreement with the computed thresholds, and the designed irregular LDPC codes outperform regular ones significantly.

A linear front end for iterative soft interference cancellation and decoding in coded CDMA

In this paper, after a description of the principles of iterative (turbo) multiuser detection for CDMA systems, a new structure, based on a linear user separator, is introduced and analyzed. The new user separator processes the matched filter outputs according to a minimum mean-square-error (MMSE) criterion for the first few iterations, while it bypasses the MMSE filter when the interferers' bits are known with some reliability. This new receiver is tested by simulation and compared with other receivers previously introduced in literature. Also, a way of studying and designing iterative multiuser receivers is given by adapting the method of EXtrinsic Information Transfer (EXIT) charts. With the EXIT charts, the receiver performance (in an information-theoretic sense) can be evaluated as it evolves along the iterations. Thus, the receiver threshold can be reliably estimated.

EXIT chart analysis of turbo multiuser detection

We study the mutual information transfer characteristics of the soft interference cancellation (SIC) multiuser detectors (MUD) for coded code-division multiple-access (CDMA) systems in synchronous additive white Gaussian noise (AWGN) and asynchronous multipath fading channels. Based on the extrinsic information transfer (EXIT) chart technique, we compare the asymptotic (as the block length goes to infinity) performance and the convergence behavior of different joint detection schemes which are highly dependent upon different system parameters and channel conditions. A framework of using the EXIT chart to analyze the coding-spreading tradeoff is also addressed for turbo multiuser detection. Results show that the SIC-minimum mean square error (MMSE) MUD generally provides a higher spectral efficiency than the SIC-match filter (MF) MUD. Moreover, the impact of finite block length on the convergence behavior of turbo MUD is also analyzed using a consistent Gaussian approximation on the distribution of the output extrinsic information.

Iterative MIMO Turbo Multiuser Detection and Equalization for STTrC-Coded Systems with Unknown Interference

Iterative multiuser detection in a single-carrier broadband multiple-input multiple-output (MIMO) system is studied in this paper. A minimum mean squared error (MMSE) low-complexity multiuser receiver is derived for space-division multiple-access (SDMA) space-time trellis-coded (STTrC) systems in frequency-selective fading channels. The receiver uses MMSE filtering to jointly detect several transmit antennas of the user of interest, while the interference from the undetected transmit antennas, cochannel interference (CCI), and intersymbol interference (ISI) are all cancelled by the soft cancellation. The performances of two extreme receiver cases are evaluated. In the first case, only one transmit antenna of the user of interest is detected at a time and the remaining ones are cancelled by soft cancellation. In the second case, all the transmit antennas are detected jointly. The comparison of the two cases shows improvement with the latter one, both in single-user and multiuser communications and in the presence of unknown cochannel interference (UCCI). It is further shown that in the multiuser case, the proposed receivers approach the corresponding single-user bounds. The number of receive antenna elements required to achieve single-user bound is thereby equal to the number of users and not to the total number of transmit antennas.

Coding-Spreading Tradeoff in LDPC-Coded CDMA With Turbo Multiuser Detection

We consider the problem of coding-spreading tradeoff in code-division multiple-access (CDMA) systems employing multiuser joint decoding. In particular, we assume the users employing the capacity-achieving low-density parity-check (LDPC) codes and at the receiver, turbo multiuser detection is used to implement joint decoding. We solve the coding-spreading optimization based on the extrinsic information signal-to-noise ratio (SNR) evolution curves for the soft-input soft-output (SISO) multiuser detectors and the SISO LDPC decoders. Two types of SISO multiuser detectors are treated, namely, the soft interference cancellation minimum mean-square-error (SIC-MMSE) detector and the soft interference cancellation matched filter (SIC-MF) detector. Moreover, both single-cell and multicell scenarios are considered. For each of these cases, we are able to characterize the extrinsic information SNR analytically, for both finite-size systems and for the so-called large systems where asymptotic performance results must be evoked. Our analysis indicates that the SIC-MMSE-based system offers a significant gain in spectral efficiency compared with the SIC-MF counterpart, in both single-cell and multicell scenarios. This is in contrast to the single-user decoding case, where it has been shown that the MMSE detector offers little advantage over the conventional matched-filter in terms of capacity in multicell scenario. Moreover, the results on coding-spreading tradeoff for finite-size systems and large systems match very well.

Packet Data Communications Over Coded CDMA Part I: Achievable Spectral Efficiency of CDMA Systems With Turbo Codes and Linear Multiuser Receivers

In this two part paper, we consider a packet data communication over code-division multiple-access (CDMA) system. As a preliminary, this part of the paper highlights the practically achievable spectral efficiency of a CDMA system with turbo codes and multiuser detection, assuming the use of hybrid type-I automatic-repeat-request. We also investigate the effect of code rates on the performance of systems with matched filter and linear minimum mean square error (MMSE) receivers, respectively. For a given ensemble distance spectra of punctured turbo codes, we use an improved union bound to evaluate the error probability of maximum likelihood turbo decoder with matched filter receiver and with linear MMSE receiver front-end, and then compute the corresponding throughput and spectral efficiency as a function of system load. The results show that with 1/3 rate turbo code and matched filter receiver a spectral efficiency of around 1.1 b/s/Hz can be obtained, while, by employing a linear MMSE receiver, the spectral efficiency can be improved further up to 1.6 b/s/Hz. The results also show that the spectral efficiency with a linear MMSE receiver saturates at a code rate of about 4/5, i.e., does not improve further with lower rate coding, while, for matched filter receivers, better performance can be obtained with lower code rate. Theoretically, as the system load goes to infinity, the spectral efficiency of linear MMSE receiver converges to that of matched filter receiver. In a practical situation, however, there exists a noticeable performance gap between the two receivers unless a very low rate code is used.